HYDRO IDS. 27 



tion will suffice for the production of a new individual ; a single ten- 

 tacle will produce a flower and stem, and finally a whole colony. A 

 transverse section of the stem will produce a flower at the distal end, 

 and a continuation of the stem, with the process by which it attaches 

 itself, at the proximal end of the section. Just so far it shows orien- 

 tation that the stem has a distal and proximal end. There is no 

 sign of bilaterality in most species, and in others the indication is so 

 slight that it is hardly worthy of the name. This development of the 

 flower always at the distal, and of the stem always at the proximal, 

 extremity of the section, shows conclusively that the stem grows 

 both ways, and that in every segment there exists a neutral plane 

 midway between both ends. 



Besides these plant-like modes of reproduction, hydroids are gen- 

 erated, like the actiniae, by spontaneous fission, a development of one 

 individual into two or more by a natural vertical cleavage. 



They multiply by ova, by ovules, by independent ciliated embryos, 

 like the lower invertebrates, the reptiles, and birds. Some varieties 

 possess a sort of marsupial pouch, in which the undeveloped young 



Fig. 9. Plumularia falcata. (Magnified.) 



are retained till they attain maturity ; and, like the mammals, in some 

 cases, the individual quits the parent after attaining perfect develop- 

 ment. Added to all these modes of reproduction, in which the analo- 

 gy must not be pressed too closely to those of higher organisms, they 

 possess two very curious modes of their own ; one given by Allman 

 in his monograph, the other by Carpenter in the latest edition of 

 " The Microscope, and its Revelations." The Tuhularian and Cam- 

 panularian hydroids, Allman tells us, develop upon their stems bell- 

 shaped medusa? (Figs. 4, 5, 11), which free themselves and swim the 

 adjacent waters. All free-swimming medusae have not yet been traced 

 to hydroid stems ; but, as all which have been carefully studied through 

 their life-history are found to originate there, it is supposed to be true 

 of the others. 



